In order to correct defocusing caused by the vertical motion due to warping of a disk-like recording medium (in the following, referred to as a disk) and tracking deviation caused by decentering of the disk, an objective lens driving device drives an objective lens in two axial directions, namely, an optical axis direction perpendicular to the surface of the recording medium (a focusing direction) and a radial direction parallel with the recording medium surface (a tracking direction), thus optically recording information on or reproducing information from the disk.
In the following, an example of a conventional objective lens driving device mentioned above will be described, with reference to the accompanying drawings. FIG. 9 is a perspective view showing a configuration of a conventional objective lens driving device 90. The objective lens driving device 90 includes a lens holder 52, to which an objective lens 51, a focusing coil 53 and a tracking coil 54 are fixed. These lens holder 52, the objective lens 51, the focusing coil 53 and the tracking coil 54 constitute a movable portion.
The focusing coil 53 has a winding axis along the optical axis direction of the objective lens 51 shown in FIG. 9. The tracking coil 54 has a winding axis along a circumferential direction, which is a direction perpendicular to the optical axis direction and a radial direction of the disk shown in FIG. 9.
The objective lens driving device 90 is provided with four support members 58, 59, 60 and 61 formed of an elastic material such as phosphor bronze sheet. One end of each of the support members 58, 59, 60 and 61 is joined to the lens holder 52, while the other end thereof is fixed to a fixing member 55.
Since the lens holder 52 is cantilevered by the support members 58, 59, 60 and 61 as described above, it is freely movable within the elastic deformation range of the support members 58, 59, 60 and 61.
Magnets 56 and 57 are fixed to yoke portions 62a provided in a base 62. The magnets 56 and 57 are disposed so that their opposite poles face each other, and the focusing coil 53 and the tracking coil 54 are disposed in a gap between the magnets 56 and 57 and constitute a focusing driving system and a tracking driving system, respectively.
The fixing member 55 holds viscoelastic members 63. The vicinity of each end portion of the support members 58, 59, 60 and 61 on the side of the fixing member 55 is covered with the viscoelastic members 63.
Now, an operation of the objective lens driving device 90 constituted as above will be described. The following description is directed to the operation of the objective lens driving device 90 that drives the objective lens 51 along two axes of the optical axis direction and the radial direction in order to correct the defocusing caused by the vertical motion due to the disk warping and the tracking deviation caused by decentering.
Referring to FIG. 9, the magnets 56 and 57 generate a magnetic flux along the circumferential direction in the gap between these magnets. When an electric current is passed through the focusing coil 53, a force along the optical axis direction acts on the focusing coil 53, which crosses this magnetic flux. The force acting on the focusing coil 53 flexes the support members 58 to 61, thereby translating the movable portion substantially along the optical axis direction.
Similarly, when an electric current is passed through the tracking coil 54, a force along the radial direction acts on the tracking coil 54, which crosses the magnetic flux. The force acting on the tracking coil 54 flexes the support members 58, 59, 60 and 61, thereby translating the movable portion constituted by the lens holder 52, the objective lens 51, the focusing coil 53 and the tracking coil 54 substantially along the radial direction.
In this way, the objective lens driving device 90 has a cantilevered configuration in which the movable portion is fixed to the front end of the support members 58, 59, 60 and 61 formed of the elastic material. Thus, various resonance occurs when the movable portion is driven by the focusing driving system and the tracking driving system.
In order to reduce this resonance, the viscoelastic members 63 are held in the fixing member 55, and the vicinities of end portions of the support members 58, 59, 60 and 61 on the side of the fixing member 55 are brought into contact with these viscoelastic members 63. In this way, the vibrations of the support members 58, 59, 60 and 61 at the time of resonance are transmitted to the viscoelastic members 63, so that the resonance is reduced by a vibration damping function of the viscoelastic members 63.
However, with respect to displacement frequency characteristics when the movable portion is driven along the radial direction (tracking direction), undesired resonance occurs in the vicinity of the oscillation frequencies of the movable portion, i.e., 3 to 4 kHz as shown in FIG. 10.
In FIG. 11, a solid line indicates how the support members 58 and 59 are displaced at this time of resonance in an enlarged view. With a miniaturization of objective lens driving devices, it has become more difficult to enlarge portions where the support members contact the viscoelastic members 63. Accordingly, as shown in FIG. 11, the amplitude amount in the portions where the support members contact the viscoelastic members 63 are small when the resonance occurs in the support members, making it difficult to reduce the resonance by the vibration damping function of the viscoelastic members 63. Furthermore, when such undesired resonance occurs, the control for driving the objective lens becomes unstable. This brings about phenomena such as tracking deviation, leading to a problem of signals not being recorded or reproduced stably.
It is an object of the present invention to provide an objective lens driving device that can reduce the resonance of support members, thus driving an objective lens in a stable manner.